1. Field of the Invention
The present invention is related to compound semiconductor growth and device fabrication. More particularly the invention relates to the growth and fabrication of planar, m-plane gallium nitride (GaN) films by performing a direct growth of the planar m-plane GaN films by hydride vapor phase epitaxy, optionally followed by a lateral epitaxial overgrowth of the GaN films to achieve reduced dislocation density.
2. Description of the Related Art
(Note: The specification of this application references various publications. The full citation of each of these publications can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The usefulness of gallium nitride (GaN) and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN) has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially by growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), or hydride vapor phase epitaxy (HVPE).
GaN and its alloys are most stable in the hexagonal würtzite crystal structure, in which the structure is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. FIG. 1 is a schematic of a generic hexagonal würtzite crystal structure 100 and planes of interest 102, 104, 106, 108 with these axes 110, 112, 114, 116 identified therein, wherein the fill patterns are intended to illustrate the planes of interest 102, 104 and 106, but do not represent the materials of the structure 100. Group III and nitrogen atoms occupy alternating c-planes along the crystal's c-axis. The symmetry elements included in the würtzite structure dictate that III-nitrides possess a bulk spontaneous polarization along this c-axis. Furthermore, as the würtzite crystal structure is non-centrosymmetric, würtzite nitrides can and do additionally exhibit piezoelectric polarization, also along the crystal's c-axis. Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction cause spatial separation of electrons and holes that in turn give rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.
One possible approach to eliminating the spontaneous and piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on non-polar planes of the crystal. Such planes contain equal numbers of Ga and N atoms and are charge-neutral. Furthermore, subsequent non-polar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction. Two such families of symmetry-equivalent non-polar planes in GaN are the {11 20} family, known collectively as a-planes, and the {1 100} family, known collectively as m-planes.
Indeed, (Al,Ga,In,B)N quantum-well structures employing nonpolar growth directions, e.g. the <11 20> a-direction or <1 100> m-direction, have been shown to provide an effective means of eliminating polarization-induced electric field effects in würtzite nitride structures since the polar axis lies within the growth plane of the film, and thus parallel to heterointerfaces of quantum wells. In the last few years, growth of nonpolar (Al,Ga,In)N has attracted great interest for its potential use in the fabrication of nonpolar electronic and optoelectronic devices. Recently, nonpolar m-plane AlGaN/GaN quantum wells grown on lithium aluminate substrates via plasma-assisted MBE and nonpolar a-plane AlGaN/GaN multi-quantum wells (MQWs) grown by both MBE and MOCVD on r-plane sapphire substrates showed the absence of polarization fields along the growth direction. More recently, Sun et al. [Sun et al., Appl. Phys. Lett. 83 (25) 5178 (2003)], and Gardner et al. [Gardner et al., Appl. Phys. Lett. 86, 111101 (2005)], heteroepitaxialy grew m-plane InGaN/GaN quantum well structures by MBE and MOCVD, respectively. Chitnis et al. [Chitnis et al., Appl. Phys. Lett. 84 (18) 3663 (2004)], grew a-plane InGaN/GaN structures by MOCVD. Most significantly, researchers at UCSB [Chakraborty et al., Appl. Phys. Lett. 85 (22) 5143 (2004)] very recently demonstrated the significant benefits of growth of reduced defect density a-plane InGaN/GaN devices utilizing low defect-density HVPE grown a-plane GaN templates. This body of literature has established that nonpolar III-nitride light emitting diodes (LEDs) and laser diodes (LDs) have the potential to perform significantly better compared to their polar counterparts.
Bulk crystals of GaN are not available so it is not possible to simply cut a crystal to present a surface for subsequent device regrowth. All GaN films are initially grown heteroepitaxially, i.e. on foreign substrates that provide a reasonable lattice match to GaN. In recent years, a number of groups have found it possible to utilize HVPE as a means of heteroepitaxially depositing GaN films that are thick enough (>200 μm) to remove the foreign substrate, yielding a free-standing GaN substrate that may then be used for homoepitaxial device regrowth. HVPE has the advantage of growth rates that are one to two orders of magnitude greater than that of MOCVD and as many as three orders of magnitude greater than MBE, an advantage that makes it an attractive technique for substrate fabrication.
One significant disadvantage of heteroepitaxial growth of nitrides is that structural defects are generated at the interface between the substrate and epitaxial film. The two predominant types of extended defects of concern are threading dislocations and stacking faults. The primary means of achieving reduced dislocation and stacking fault densities in polar c-plane GaN films is the use of a variety of lateral overgrowth techniques, including lateral epitaxial overgrowth (LEO, ELO, or ELOG), selective area epitaxy, and PENDEO® epitaxy. The essence of these processes is to block or discourage dislocations from propagating perpendicular to the film surface by favoring lateral growth over vertical growth. These dislocation-reduction techniques have been extensively developed for c-plane GaN growth by HVPE and MOCVD.
Only recently have GaN lateral growth techniques been demonstrated for a-plane films. Craven, et al. [Craven et al., Appl. Phys. Lett. 81 (7) 1201 (2002)], succeeded in performing LEO using a dielectric mask on a thin a-plane GaN template layer via MOCVD. Our group [Haskell et al., Appl. Phys. Lett. 83 (4) 644 (2003)] subsequently developed a LEO technique for the growth of a-plane GaN by HVPE. However, to date no such process has been developed or demonstrated for m-plane GaN.
The present invention overcomes these challenges and for the first time provides a technique for the growth of high-quality m-plane GaN by HVPE.